Help with Random Motion

Hey all, I’m a newbie to Flash and these forums.

I’ve been following Kirupa’s Random Motion tutorial, available here

I have managed to get it to work. However I wish to mask the effect. I have placed a mask, with my layer with the random motion elements underneath it. However the mask does not work.

If anyone has read or done that tutorial, can you suggest a reason why I cannot mask the motion?

Thankyou for any help.